1. Field of the Invention
The present invention relates to a compound for a rare-earth bonded magnet and a rare-earth bonded magnet using such a compound.
2. Description of the Related Art
A bonded magnet is currently used in various types of electric equipment including motors, actuators, loudspeakers, meters and focus convergence rings. A bonded magnet is a magnet obtained by mixing together an alloy powder for a magnet (i.e., a magnet powder) and a binder (such as a resin or a low-melting metal) and then compacting and setting the mixture.
In the prior art, an Fe—R—B based magnet powder available from Magnequench International Inc. (which will be referred to herein as “MQI Inc.”), or a so-called “MQ powder”, is used extensively as a magnet powder for a bonded magnet. The MQ powder normally has a composition which is represented by the general formula: Fe100-a-bBaRb (where Fe is iron, B is boron, and R is at least one rare-earth element selected from the group consisting of Pr, Nd, Dy and Tb). In this general formula, a and b satisfy the inequalities 1 at %≦a≦6 at % and 10 at %≦b≦25 at %, respectively. The MQ powder is a rare-earth alloy powder with a high R mole fraction b.
A conventional alloy powder for a bonded magnet such as the MQ powder is obtained by rapidly cooling and solidifying a molten material alloy (i.e., “molten alloy”). As such a melt quenching process, a single roller method (typically, a melt spinning process) is often used. The single roller method is a method of cooling and solidifying a molten alloy by bringing the alloy into contact with a rotating chill roller. In this method, the resultant rapidly solidified alloy has the shape of a thin strip (or ribbon), which is elongated in the surface velocity direction of the chill roller. The thin-strip rapidly solidified alloy obtained in this manner is thermally treated and then pulverized to a mean particle size of 300 μm or less (or typically about 150 μm) to be a rare-earth alloy powder for a permanent magnet. In the following description, the rare-earth alloy powder obtained by such a melt quenching process will be simply referred to herein as a “conventional rapidly solidified magnet powder”, which will not include the nanocomposite magnet powder to be described later.
By mixing the conventional rapidly solidified magnet powder with a resin (which will include herein rubber or elastomer), a compound for a bonded magnet (which will be simply referred to herein as a “compound”) is prepared. An additive such as a lubricant or a coupling agent is sometimes mixed with this compound.
Thereafter, by compacting this compound into a desired shape by a compression, extrusion or injection molding process, for example, a bonded magnet is obtained as a compact for a permanent magnet (which will be sometimes referred to herein as a “permanent magnet body”). Also, the bonded magnet to be obtained by the compression or extrusion process includes the binder at a relatively low percentage, and may be further subjected to a surface treatment to protect the magnet powder from corrosion.
Meanwhile, an iron-based rare-earth alloy (e.g., Fe—R—B based, in particular) nanocomposite magnet (which is sometimes called an “exchange spring magnet”) powder has recently been used more and more often as a magnet powder for a bonded magnet because such a magnet powder is relatively cost effective. The Fe—R—B based nanocomposite magnet is an iron-based alloy permanent magnet in which nanometer-scale crystals of iron-based borides (e.g., Fe3B, Fe23B6 and other soft magnetic phases) and those of an R2Fe14B phase as a hard magnetic phase are distributed uniformly within the same metal structure and are magnetically coupled together via exchange interactions (see Japanese Laid-Open Publication No. 2001-244107, for example).
The nanocomposite magnet includes soft magnetic phases and yet exhibits excellent magnetic properties due to the magnetic coupling (i.e., the exchange interactions) between the soft and hard magnetic phases. Also, since there are those soft magnetic phases including no rare-earth elements R such as Nd, the total percentage of the rare-earth elements R can be relatively low (a typical R mole fraction is 4.5 at %). This is advantageous for the purposes of reducing the manufacturing cost of magnets and supplying the magnets constantly. Furthermore, since R, which is active to oxygen, is included at a low percentage, the magnet also excels in anticorrosiveness. The nanocomposite magnet may also be made by a melt quenching process. Then, the nanocomposite magnet is pulverized by a predetermined method to obtain a nanocomposite magnet powder.
However, the conventional compound for a rare-earth bonded magnet, made of the alloy powder (or magnet powder) described above, has the following drawbacks.
Firstly, to obtain a uniform microcrystalline structure, which contributes to expressing excellent magnetic properties, for the conventional rapidly solidified magnet powder (e.g., the MQ powder), the molten alloy needs to be rapidly cooled and solidified at a relatively high rate. For example, when the conventional rapidly solidified magnet powder is made by a single roller method, the roller should have a surface velocity of 20 m/s or more to obtain a rapidly solidified alloy (typically in a thin strip shape) with a thickness of 50 μm or less (typically, 20 μm to 40 μm).
However, when the rapidly solidified alloy obtained in this manner is pulverized, the resultant powder will mostly consist of particles with aspect ratios of less than 0.3. If a compound for a rare-earth bonded magnet (which will be simply referred to herein as a “compound”) is obtained by mixing the powder of such a shape and a binder together, the compound will exhibit poor flowability during an injection molding process, for example. Thus, such a compound may need to be compacted at a higher temperature and/or at a higher pressure, the types and applications of resins to be used may be limited, or the content of the magnet powder may be limited to ensure sufficient flowability. Also, it has been difficult to obtain a bonded magnet having a complex shape or a bonded magnet to fill a small gap (e.g., with a width of 2 mm) as in an IPM (interior permanent magnet) type motor including a magnet embedded rotor as disclosed in Japanese Laid-Open Publication No. 11-206075. As used herein, the “aspect ratio” means the ratio of the minor-axis size of a particle relative to the major-axis size thereof.
Furthermore, in a compound including the conventional rapidly solidified magnet powder (e.g., the MQ powder), the magnet powder is easily oxidized in air, the properties of the magnet powder itself deteriorate due to the heat during an injection molding process, and the resultant bonded magnet may exhibit insufficient magnetic properties. The present inventors discovered via experiments that such oxidation was particularly noticeable when the conventional rapidly solidified magnet powder included particles with particle sizes of 53 μm or less.
Accordingly, when a bonded magnet is made of a compound including the conventional rapidly solidified magnet powder, the compacting temperature is limited to minimize the oxidation to be caused by the heat during the molding process. As a result, the compactibility, including the flowability, must be sacrificed.
Furthermore, as for a compound to be subjected to an injection molding process or an extrusion process, the compound being prepared is exposed to the heat that is applied to melt a thermoplastic resin as a binder. Thus, during the manufacturing process, the magnet powder in the compound may be oxidized and the resultant magnetic properties may deteriorate.
Furthermore, when the injection-molded body is cut off from the runner portion, the magnet powder will be exposed on the resin surface on the cross section of the molded body. Also, the magnet powder itself may be exposed on the cross section. When the magnet powder is exposed in some areas in this manner, corrosion easily advances from those areas. This problem is particularly noticeable when the wettability between the resin and the magnet powder is poor. Also, it depends not only on a particular material combination but also on how the process step of mixing the resin and magnet powder material is carried out. Specifically, the conventional rapidly solidified magnet powder has a small aspect ratio, and is hard to mix uniformly in the compound preparing process step. Thus, in the resultant compound, the wettability between the resin and the magnet powder may be poor enough to expose the magnet powder particles here and there. Furthermore, since the magnet powder has a large aspect ratio, the magnet powder is crushed by shear force applied in the mixing process step to newly expose other cross sections easily. As a result, the magnet powder in the resultant compound is easily oxidizable.
On the other hand, the conventional Fe—R—B based nanocomposite magnet powder includes the rare-earth elements at a relatively low mole fraction and typically includes 30 vol % or less of hard magnetic phases. Thus, the magnetic properties (e.g., coercivity HcJ) thereof are inferior to those of the conventional rapidly solidified magnet powder (such as the MQ powder). Accordingly, it is difficult to make a bonded magnet with sufficient magnetic properties from a compound including only the nanocomposite magnet powder as its magnet powder. For example, a bonded magnet for use in a motor for a hard disk drive (HDD) could not be made from such a nanocomposite magnet powder. For that reason, the conventional nanocomposite magnet powder described above needs to be mixed with the conventional rapidly solidified magnet powder. Consequently, it has been difficult so far to obtain a bonded magnet with excellent magnetic properties while totally eliminating the problems of the compound including the conventional rapidly solidified magnet powder.